>>2949606
It's a piece of elastic sewn to the back panel with a single straight stitch. It is in between two layers of fabric. Just pick out the thread carefully with a seam ripper. The elastic will stay inside unless you destitch the top piping and pull it out. You would then have to resew the top piping. I'd just leave it how it is, without the elastic there the bikini bottom will become too long and ride too high up on the waist. You probably won't be happy with it even if you successfully remove it.